In terms of sheer quantity, carbon dioxide is society's largest contribution to global warming, but there are some lesser-known gases that also jeopardize the Earth's climate future. This list includes methane, nitrous oxide, chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs) and several others. These gases may make up a small percentage of the emissions society generates, but they pack a devastating punch when released.

The threat level for each of these gases varies based on several factors, most notably their lifetime in the atmosphere and their potential to influence global warming . Reducing the emissions of those gases in addition to those from carbon dioxide is critical to achieving a stable climate.

Taking action against greenhouse-gas emissions

For decades, climate-change discussions have centered on actions to curb carbon-dioxide emissions. Now, government officials are beginning to focus their attention on reducing other types of harmful gases, including HFC emissions. HFCs are commonly used as refrigerants and propellants in aerosols.

Currently, HFCs comprise only 2 percent of total carbon-dioxide-equivalent emissions, but this percentage may increase to as much as 20 percent if society continues on its current emissions trajectory. Alternatively, a global phasedown of HFCs could avoid 100 gigatons of carbon-dioxide-equivalent emissions by 2050, and prevent a global average temperature increase of 0.5 degrees Celsius (0.9 degrees Fahrenheit) by 2100, according to findings announced in Bangkok, Thailand, in June by members of the air-conditioning and refrigeration industry at the Advancing Ozone and Climate Protection Technologies: Next Steps conference.

Several nations are already taking action: the U.S., Canada and Mexico proposed an amendment in April to the Montreal Protocol that would gradually phase down the production and consumption of 19 HFC substances. More than 100 countries already support such an amendment. While the requirement to limit HFC emissions has yet to be adopted, some countries are voluntarily pledging to reduce HFC production and consumption on their own.

In June, President Barack Obama and President Xi Jinping of China announced a cooperative commitment to reduce emissions of HFCs. Because the United States is the largest consumer of HFCs and China is the largest producer of them, this commitment could have an enormous impact on reducing global warming. Obama and Xi plan "to use the expertise and institutions of the Montreal Protocol" to phase down the use of hydrofluorocarbons. After reducing carbon-dioxide emissions from power plants, the phasing-down of HFCs is the next-biggest step the United States can take toward achieving its goal of reducing greenhouse-gas emissions by 17 percent by 2020, according to the World Resources Institute.

Why are HFCs so bad?

HFCs, which belong to a category of substances known as short-lived climate forcers (SLCFs), have an incredibly high potential to contribute to global warming, yet a relatively short atmospheric lifetime.

The threat level of a gas is determined, in part, by its global-warming potential (GWP), a measurement of how much heat a gas can trap in the atmosphere. It is expressed as a ratio of a gas's heat-trapping ability relative to that of carbon dioxide (which has a GWP standardized at one), and is often expressed over a 100-year timescale.

Many greenhouse gases have much higher GWPs than carbon dioxide. CFCs — which are most widely used as refrigerants, propellants and solvents (and are better known for their impact on the Earth's ozone layer, which protects the planet from ultraviolet radiation) — have GWPs in the thousands. That means that even miniscule CFC emissions can severely impact the atmosphere. Similarly, HFCs have GWPs ranging from 140 to 11,700, according to the U.S. Environmental Protection Agency. Therefore, avoiding HFC emissions will make a huge difference in tackling climate change.

However, it's critical to consider atmospheric lifetime, or the average length of time a molecule of gas exists in the atmosphere before being converted into another chemical compound or absorbing back into a "sink," like a forest or ocean. Atmospheric lifetimes vary greatly, with SLCFs remaining in the atmosphere for weeks, months or years. (Some substances can last for centuries or millennia.) Eliminating HFCs and other SLCFs will help decelerate climate change in the immediate future, since they only linger in the atmosphere for a brief period of time once emissions are curbed. According to the Center for Climate and Energy Solutions, SLCFs may be responsible for as much as 40 percent of human-induced climate change, which means that reducing their emissions could have a considerable impact on the Earth's climate future.

The Montreal Protocol success

Scientists began to study the weakening of Earth's ozone layer in the 1970s, and realized CFCs are extremely damaging to the ozone layer. At that time, the focus was on protecting the ozone layer, and less so on the potential damages of gases with high global-warming potential . In 1989, the Montreal Protocol called for a phaseout of several ozone-depleting substances, namely CFCs and hydrochlorofluorocarbons (HCFCs). The protocol, signed by 46 countries, stipulated that the production and consumption of certain CFCs by participating countries would decline to zero by 1996.

The Montreal Protocol is an unmitigated success at a time when other international climate agreements have been weak or nonexistent. All of the United Nations member nations now comply with it, which has led to the effective phaseout of 97 percent of all ozone-depleting substances, the equivalent of more than 200 gigatons of greenhouse gases and nearly six years' worth of total global emissions.

However, the phaseout of CFCs caused many industries to turn to HFCs as a replacement, since HFCs are not harmful to the ozone layer. Researchers have now come to realize the unfortunate dangers of HFCs and their extremely high global-warming potential. As discussed in the new U.S.-China agreement on HFCs, implementing a framework similar to the Montreal Protocol will hopefully lead to a rapid reduction in HFC emissions. Because HFCs are short-lived but powerful climate forcers, limiting their use now will result in environmental benefits within this lifetime.

The views expressed are those of the author and do not necessarily reflect the views of the publisher. This article was originally published on LiveScience.com.